1. Field of the Invention
This invention relates to a method of recording time code data. This invention also relates to an apparatus for recording time code data.
2. Description of the Related Art
Time-information representing data of a given code is referred to as time code data or time code information. In some VTR's (video tape recorders), time code data is periodically added to or superimposed on a video signal, and the time-code-added video signal is recorded on a magnetic tape. Generally, time code data is used as an indication of absolute time related to each frame represented by a video signal. The time code data can also be used in recognizing absolute positions of segments of the video signal on the magnetic tape. Furthermore, the time code data can be used as a reference for providing synchronization when video signals reproduced from different magnetic tapes are edited into a single video signal.
Different words or different states of time code data are assigned to respective frames represented by a video signal. The time code data is incremented by "1" each time a frame represented by the video signal is replaced by a next frame. Accordingly, the time code data indicates an order number of every frame related to the video signal.
A known helical-scan digital VTR is able to record a digital signal on a magnetic tape at a data rate of 14.1 Mbps. During the recording of the digital signal, 60 slant tracks are formed on the magnetic tape per second. The known digital VTR can reproduce a digital signal from a magnetic tape at a data rate of 14.1 Mbps or a data rate smaller than 14.1 Mbps. For example, either a data rate of 4.6 Mbps or a data rate of 2 Mbps is set as the smaller data rate. It should be noted that the reproduction of a digital signal at a data rate of 4.6 Mbps corresponds to 1/3-speed playback while the reproduction at 2 Mbps corresponds to 1/7-speed playback.
In conceivable digital broadcasting, a source digital signal of video information and audio information has a bit rate of 4.6 Mbps, and the source digital signal is compressed into a second digital signal having a bit rate of 13.8 Mbps before the second digital signal is broadcasted. In this case, a program represented by the source digital signal is transmitted in one third of a normal time. It is assumed that the received second digital signal has been recorded by a digital VTR. When the second digital signal is reproduced at a speed equal to one third of the recording speed, the source digital signal which has a bit rate of 4.6 Mbps is properly recovered.
As previously explained, in the known digital VTR, during the recording of a digital video signal, 60 slant tracks are formed on a magnetic tape per second. In the case where a digital video signal to be recorded has a frame frequency of 30 Hz, every frame represented by the digital video signal is assigned to a pair of two neighboring tracks. In this case, time code data representing a same value (a same number) corresponds to two neighboring tracks. During high-speed playback, it is possible to search for desired absolute time, that is, the position of a desired 1-frame-corresponding segment of the digital video signal on the magnetic tape, by referring to the reproduced time code data.
It is assumed that a source digital video signal with a frame frequency of 30 Hz is compressed into a second digital video signal having three times the original bit rate, and the second digital video signal is recorded by the known digital VTR. In this case, three frames represented by the source digital video signal are assigned to two tracks, and hence time code data representing a same value (a same number) does not correspond to two neighboring tracks. Thus, during high-speed playback, it tends to be difficult to search for the position of a desired 1-frame-corresponding segment of the second digital video signal on the magnetic tape by referring to the reproduced time code data in the normal way.